1. Field of the Invention
This invention relates to drilling drain holes in the earth. More particularly, method and apparatus are provided for drilling through formations containing carbonate minerals using an acidic drilling fluid, a very flexible tubular and a micro-jet bit.
2. Description of Related Art
It is estimated that sixty to seventy percent of the oil and gas reserves in the world are in reservoirs having a predominance of carbonate minerals (limestone or dolomite). It is also estimated that over sixty percent of the original oil-in-place remains after traditional methods of oil production are exhausted. A large part of this oil is left in reservoirs because it is in rock that is not in adequate hydraulic communication with a well.
The typical procedure to produce oil or gas from a carbonate mineral formation is to drill a vertical well, place casing in the well, place cement between the casing and the formation, and perforate the casing. It is common to pump acid (usually 15% hydrochloric acid) through the perforations to improve fluid communication between the well and the formation. The acid may be pumped at low (matrix) rates to dissolve the rock around perforations, affecting only the region at or very near the wellbore, or it may be pumped at high rates and at a pressure above fracturing pressure to create a hydraulic fracture in the rock (acid fracturing). A single vertical hydraulic fracture extends in opposite directions away from the wellbore, in the azimuth direction determined by stress in the earth. This may not be the direction preferred to maximize recovery of hydrocarbons from the formation.
Acid in the hydraulic fracture etches the wall of the fracture, which provides a path to allow greater flow rate to the well. But earth stress tends to close the fracture and limit flow capacity of the fracture. Also, fluid flowing towards the wellbore brings insoluble components of the rock that may clog the fracture. Due to the above circumstances, the traditional acid injection procedures affect only a small portion of the reservoir. In addition, with acid fracturing there is the risk that the fracture will extend vertically into an unwanted water zone, which can make production of the well uneconomical.
To improve recovery, industry has resorted to drilling horizontal wells. These have been especially successful in naturally fractured reservoirs, where the horizontal wellbore may intersect natural vertical fractures. Horizontal wellbores may be drilled by using a directional drilling assembly to change the direction at the bottom of a vertical well as the well is drilled, forming a radius of curvature of 25 feet or more. Horizontal wells may also be formed by drilling “drainholes” out of a wellbore with a directional drilling assembly or by diverting flexible tubing and driving or pushing the tubing through the earth. These are usually expensive procedures and have a typical turning radius of twenty-five or more feet.
There is a need for apparatus and methods that are economical and reliable to drill drain holes in carbonate reservoirs so as to recover more of the resources from the formations.